1. Field of the Invention
The present invention relates to a wireless communication method and a device in which a transmittal right is acquired by conditional access control and data transmission is started.
2. Description of the Related Art
Since the past, a wireless communication system has been proposed in which road-vehicle communication and vehicle-vehicle communication can be performed over a single wireless communication channel (refer to, for example, Japanese Patent Laid-open Application No. 2007-60289).
In the proposed wireless communication system, a transmission signal from a road-side radio communication apparatus includes information on transmission timing of the road-side radio communication apparatus. An on-vehicle radio communication apparatus receiving the transmission signal acknowledges a time at which the road-side radio communication apparatus performs transmission based on the information and stops transmission from the on-vehicle radio communication apparatus during this time.
Therefore, in the wireless communication system, the on-vehicle radio communication apparatus does not interfere with the transmission from the road-side radio communication apparatus. The transmission from the road-side radio communication apparatus to the on-vehicle radio communication apparatus can be given priority and performed with certainty.
In the above-described wireless communication system, the on-vehicle radio communication apparatus stops data transmission while the road-side radio communication apparatus is transmitting data. However, when a data transmission request is generated while the on-vehicle radio communication apparatus is stopped, the on-vehicle wireless communication system enters a data transmission wait state. The on-vehicle wireless communication system starts conditional access control to promptly start data transmission when the road-side radio communication apparatus stops data transmission and a wireless communication channel becomes free.
Therefore, in the above-described wireless communication system, for example, as shown in FIG. 14, when a data transmission is time of the road-side radio communication apparatus (road-side unit [RSU]) increases and a number of on-vehicle radio communication apparatuses for which transmission requests are generated during the data transmission time increases, when the data transmission by the road-side radio communication apparatus subsequently ends, a large number of on-vehicle radio communication apparatuses in a transmission wait state until this point simultaneously start access control. As a result, a temporary excessive traffic state occurs in the wireless communication channel. Probability of transmission packet collision increases.
A problem such as this is not limited to the above-described wireless communication system. The problem similarly occurs when a wireless communication system is that in which a plurality of radio communication apparatuses share a single wireless communication channel.
In other words, in the wireless communication system, when a certain radio communication apparatus starts to transmit a packet that is significantly longer that at an ordinary time (referred to, hereinafter, as a long packet), because other radio communication apparatuses cannot transmit data during this time, the number of radio communication apparatuses entering a data transmission wait state increases.
Therefore, even in a typical wireless communication system such as this, a large number of radio communication apparatuses in the data transmission wait state simultaneously start access control when the certain radio communication apparatus completes transmission of the long packet. In a similar manner as that described above, the temporary excessive traffic state occurs. The probability of transmission packet collision increases.